The present invention relates generally to high strength, light weight shaft members and a method of fabrication. More particularly, it relates to filament reinforced composite structures in the form of shafts which shafts have very high strength in relation to the weight thereof.
The fabrication of filament reinforced composite structures having high strength ceramic fibers embedded within metal matrices are known.
The preparation of titanium alloy base foils, sheets, and similar articles and of reinforced structures in which silicon carbide fibers are embedded in a titanium base alloy are described in U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337; assigned to the same assignee as the subject application. The texts of these patents are incorporated herein by reference. Preparation of composites as described in these patents is the subject of intense study inasmuch as the composites have very high strength properties in relation to their weight. One of the properties which is particularly desirable is the high tensile properties imparted to the structures by the high tensile properties of the silicon carbide fibers or filaments. The tensile properties of the structures are related to the rule of mixtures. According to this rule the proportion of the property, such as tensile property, which is attributed to the filament, as contrasted with the matrix, is determined by the volume percent of the filament present in the structure and by the tensile strength of the filament itself. Similarly, the proportion of the same tensile property which is attributed to the matrix is determined by the volume percent of the matrix present in the structure and the tensile strength of the matrix itself.
Prior to the development of the processes described in the above-referenced patents, such structures were prepared by sandwiching the reinforcing filaments between foils of titanium base alloy and pressing the stack of alternate layers of alloy and reinforcing filament until a composite structure was formed. However, that prior art practice was found to be less than satisfactory when attempts were made to form specific structures of novel shape in which the filament was an internal reinforcement for the entire structure.
One of the structures which was formed by use of the prior art foil and fiber technique described immediately above is a shaft structure. Because of the fiber reinforcement and the use of lighter weight matrix metal, such as titanium, and due to the design of the shaft as a hollow article, the shaft could be fabricated employing reinforced deposit metal with lighter weight and greater stiffness than was possible employing metal alone.
One of the difficulties encountered in forming such prior art hollow shaft members using the prior art foil and fiber technique was that the volume of empty space in the composite prior to consolidation was quite large and because of this larger portion of empty space in the precompacted composite a substantial movement of the material being compacted was necessary in order to achieve the desired full density of the consolidated article. In order to achieve such full density, the foil had to flow in between and essentially around the individual fibers as the consolidation took place as, for example, through HIPing. However, as explained more fully below, the preparation of such hollow shaft members through practice of the present invention, greatly minimizes or eliminates the amount of void space in the compacted structure and thus overcomes the problems associated with the need for substantial movement of the fiber and matrix material during consolidation.
The structures taught in the above-referenced patents and the methods by which they are formed, greatly improve over the earlier practice of forming sandwiches of matrix and reinforcing filament by compression.
Later it was found that while the structures prepared as described in the above-referenced patents have properties which are a great improvement over earlier structures, the attainment of the potentially very high ultimate tensile strength of these structures did not measure up to the values theoretically possible when certain metals were employed as the matrix material. The testing of composite formed according to the methods taught in the above patents has demonstrated that, when employing Ti1421 alloy, although modulus values are generally in good agreement with the rule of mixtures predictions, the ultimate tensile strength is usually much lower than predicted by the underlying properties of the individual ingredients to the composite. A number of applications have been filed which are directed toward overcoming the problem of lower than expected tensile properties and a number of these applications are copending. These include applications Ser. No. 445,203, filed Dec. 4 1989; Ser. No. 459,894, filed Jan. 2, 1990, now U.S. Pat. No. 4,978,585; and Ser. Nos. 455,041 and 455,048, both filed Dec. 22, 1989. The texts of these applications are incorporated herein by reference.
It has been discovered that a unique structure can be formed employing monotape strips of metal matrix strengthened by filamentary reinforcement. By the term "monotape" as used herein is meant a ribbon-like structure in which a single layer of filament reinforcement is embedded within a layer of matrix metal. These novel structures are formed by winding the monolayer strips as a ribbon about an elongated mandrel having the dimensions of a shaft. The ribbon is wound about the mandrel at an angular pitch such that the ribbon completely covers the shaft and the ribbon follows a spiral path along the length of the shaft. A single ribbon may be employed so that its opposite edges abut to form a spiral seam extending along the length of the shaft shaped mandrel. Alternatively, a plurality of composite ribbons can be wound on the shaft so that their edges abut along a spiral path and the surface of the mandrel is completely covered. The angle of the spiral relative to the axis of the shaft may be of the order of 15 to 30 degrees.
In order to provide additional strengthening of the structure, layers of composite ribbon are wound on the shaft shaped mandrel with each layer wound in a reverse pitch to the next underlayer. Once the several layers of composite ribbon are wound on the mandrel, the structure is consolidated by HIPing to combine the several layers into a single consolidated tubular structure.